This actually is a trick question.
The answer is ambiguous as to what "should" happen. Sure, the C# compiler takes it out of the realm of ambiguity to the concrete; however, since these methods are overloading one another, and are neither overriding nor shadowing, it is reasonable to assume that the "best argument fit" should apply here, and therefore conclude that it is A::Foo(int n) that should be called when provided an integer as an argument.
To prove that what "should" happen is unclear, the exact same code when run in VB.NET has the opposite result:
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles Button1.Click
Dim b As New B
b.Foo(5) ' A::Foo
b.Foo(5.0) ' B::Foo
End Sub
End Class
Class A
Sub Foo(ByVal n As Integer)
MessageBox.Show("A::Foo")
End Sub
End Class
Class B
Inherits A
Overloads Sub Foo(ByVal n As Double)
MessageBox.Show("B::Foo")
End Sub
End Class
I realize that I am opening up the opportunity for the C# programmers to "bash" VB.NET for not complying with C# . But I think one could make a very strong argument that it is VB.NET that is making the proper interpretation here.
Further, IntelliSense within the C# IDE suggests that there are two overloads for the class B (because there are, or at least should be!), but the B.Foo(int n) version actually can't be called (not without first explicitly casting to a class A). The result is that the C# IDE is not actually in synch with the C# compiler.
Another way of looking at this is that the C# compiler is taking an intended overloads and turning it into a shadowed method. (This doesn't seem to be the right choice to me, but this is obviously just an opinion.)
As an interview question, I think that it can be ok if you are interested in getting a discussion about the issues here. As for getting it "right" or "wrong", I think that the question verges on a trick question that could be easily missed, or even gotten right, for the wrong reasons. In fact, what the answer to the question "should be" is actually very debatable.